callchain-overhead-calculation.txt

Overhead calculation
--------------------
The CPU overhead can be shown in two columns as 'Children' and 'Self'
when perf collects callchains (and corresponding 'Wall' columns for
wall-clock overhead).  The 'self' overhead is simply calculated by
adding all period values of the entry - usually a function (symbol).
This is the value that perf shows traditionally and sum of all the
'self' overhead values should be 100%.

The 'children' overhead is calculated by adding all period values of
the child functions so that it can show the total overhead of the
higher level functions even if they don't directly execute much.
'Children' here means functions that are called from another (parent)
function.

It might be confusing that the sum of all the 'children' overhead
values exceeds 100% since each of them is already an accumulation of
'self' overhead of its child functions.  But with this enabled, users
can find which function has the most overhead even if samples are
spread over the children.

Consider the following example; there are three functions like below.

-----------------------
void foo(void) {
    /* do something */
}

void bar(void) {
    /* do something */
    foo();
}

int main(void) {
    bar()
    return 0;
}
-----------------------

In this case 'foo' is a child of 'bar', and 'bar' is an immediate
child of 'main' so 'foo' also is a child of 'main'.  In other words,
'main' is a parent of 'foo' and 'bar', and 'bar' is a parent of 'foo'.

Suppose all samples are recorded in 'foo' and 'bar' only.  When it's
recorded with callchains the output will show something like below
in the usual (self-overhead-only) output of perf report:

----------------------------------
Overhead  Symbol
........  .....................
  60.00%  foo
          |
          --- foo
              bar
              main
              __libc_start_main

  40.00%  bar
          |
          --- bar
              main
              __libc_start_main
----------------------------------

When the --children option is enabled, the 'self' overhead values of
child functions (i.e. 'foo' and 'bar') are added to the parents to
calculate the 'children' overhead.  In this case the report could be
displayed as:

-------------------------------------------
Children      Self  Symbol
........  ........  ....................
 100.00%     0.00%  __libc_start_main
          |
          --- __libc_start_main

 100.00%     0.00%  main
          |
          --- main
              __libc_start_main

 100.00%    40.00%  bar
          |
          --- bar
              main
              __libc_start_main

  60.00%    60.00%  foo
          |
          --- foo
              bar
              main
              __libc_start_main
-------------------------------------------

In the above output, the 'self' overhead of 'foo' (60%) was add to the
'children' overhead of 'bar', 'main' and '\_\_libc_start_main'.
Likewise, the 'self' overhead of 'bar' (40%) was added to the
'children' overhead of 'main' and '\_\_libc_start_main'.

So '\_\_libc_start_main' and 'main' are shown first since they have
same (100%) 'children' overhead (even though they have zero 'self'
overhead) and they are the parents of 'foo' and 'bar'.

Since v3.16 the 'children' overhead is shown by default and the output
is sorted by its values. The 'children' overhead is disabled by
specifying --no-children option on the command line or by adding
'report.children = false' or 'top.children = false' in the perf config
file.